<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link href="test_div.css" rel="stylesheet" type="text/css">
</head>
<body style="background-color: aquamarine">
<div class="yellow_bg">
    <h4>测试1</h4>
</div>

</body>
</html>